1. Field of the Present Invention
The present invention relates to the field of testing of nuclear fuel rods and, in particular, to improvements in the testing of nuclear fuel rods by ultrasonic techniques.
2. Background Prior Art
The use of ultrasonic energy for detecting defects in nuclear fuel rods is known. U.S. Pat. Nos. 4,174,255 and 4,517,152 are two patents disclosing such use of ultrasonic energy for testing of nuclear fuel rods.
In U.S. Pat. No. 4,879,088, assigned to the same assignee as that of the present invention, nuclear fuel rods are checked for defects by ultrasonic pulses emitted from a transducer. Such fuel rods, which form the core of the reactor, are elongated elements. The rods typically have diameters of about 3/8 to 1/2 inch, are about 10 to 12 feet long and are grouped in bundles which are generally square. Each rod is formed of cladding of zirconium alloy or stainless steel, which is filled with uranium dioxide, typically in the form of pellets. The remainder of the space within the cladding is commonly filled with helium. During the operation of the reactor, holes may develop in the cladding due to stress, corrosion, wear or defective welding to the end plugs which close the ends of the cladding tubes. If this occurs, the helium and fusion gases will escape into the cooling water of the reactor and the water will enter the cladding tubes.
According to the approach of U.S. Pat. No. 4,879,088, a transducer is caused to traverse a fuel assembly consisting of a group of fuel rods. During the traverse, a series of ultrasonic pulses is emitted from the transducer in the form of a beam. When the beam strikes a fuel rod, it is reflected from the outer surface. If the beam is exactly normal to the surface, it will be reflected back into the transducer to a maximum degree which will give rise to an electrical signal ("pulse-echo" technique). Part of the ultrasonic energy penetrates the cladding but the normal rod does not pass ultrasonic energy (the internal gas and uranium dioxide are nonconductive) resulting in "wall ringing" between inner and outer walls of the rod. The wall ringing is recorded. If the rod is filled with water, there will be a transfer of the ultrasonic energy from the tubing wall into the water where it is effectively dispersed. This greatly attenuates the wall ringing. This will occur whether uranium dioxide is present or not. The attenuation of the wall ringing identifies a defective fuel rod.
In practice, test results employing the ultrasonic approach of fuel rod testing have indicated erroneous readings under certain circumstances. Such erroneous readings lead to conclusions that specific fuel rods indicate failure, when in fact they were properly functioning.
Further study of the fuel rod assembly suggests that differences in fuel rod and guide tube diameters (in the case of PWR fuel assemblies) and fuel rod bow, will cause alignment problems of the rods leading to the erroneous ultrasound readings. While U.S. Pat. No. 4,847,037 to Scharpenburg does recognize that bowing of fuel rods occurs, there is no suggestion for providing pre-alignment to all fuel rods of an assembly to correct for such bowing.